The present invention relates to nuclear reactor fuel assemblies and more particularly to a fuel assembly grid designed to improve fuel rod support, strength, and reactor performance.
It is well known that the nuclear fuel for heterogeneous nuclear reactors is in the form of fuel elements or rods which are grouped together in side-by-side assemblies or bundles. These fuel assemblies also include fixed rods containing burnable poisons and hollow tubes through which control element assemblies are arranged to pass. The liquid moderator-coolant, normally water, flows upwardly through the reactor core in channels or longitudinal passageways formed between the fuel rods and fuel assemblies.
One of the operating limitations on current reactors is established by the onset of film boiling on the surfaces of the fuel element rods. The phenomenon is commonly referred to as departure from nucleate boiling (DNB) and is affected by the fuel element spacing, system pressure, heat flux, coolant enthalpy and coolant velocity. When DNB occurs, there is a rapid rise in the temperature of the fuel element due to the reduced heat transfer, which can ultimately result in failure of the element. Therefore, in order to maintain a factor of safety, nuclear reactors must be operated at a heat flux level somewhat lower than that at which DNB occurs. This margin is commonly referred to as the "thermal margin".
Nuclear reactors normally have some regions in the core which have a higher neutron flux and power density than other regions. This situation may be caused by a number of factors, one of which is the presence of control rod channels in the core. When the control rods are withdrawn, these channels are filled with moderator which increases the local moderating capacity and thereby increases the power generated in the fuel. In these regions of high power density known as "hot channels", there is a higher rate of enthalpy rise than in other channels. It is such hot channels that set the maximum operating conditions for the reactor and limit the amount of power that can be generated because the critical thermal margin is first reached in these channels.
Attempts have been made in the past to solve these problems and increase DNB performance by incorporating integral flow deflector vanes into the support grid structures employed to contain the members of the fuel assembly. These vanes can improve performance by increasing coolant mixing and fuel rod heat transferability downstream of the vanes. These attempts to improve performance have met with varying success depending on the vane design and the design of other grid components which can impact the effectiveness of vanes. To maximize the benefit of the vanes, the size, shape, bend angle, and location of the vanes must be optimized. The vanes are especially beneficial adjacent to the hot channels. The remaining components of the grid which include the strips, rod support features and welds must be streamlined to reduce the turbulence generated in the vicinity of the vanes. Further constraints on designing the grids include minimizing grid pressure drop and maximizing grid load carrying strength.
Grids are generally of egg-crate configuration and are spaced longitudinally along the fuel assembly to provide support for the fuel rods, maintain fuel rod spacing, promote mixing of coolant, provide lateral support and positioning for fuel assembly guide tubes, and provide lateral support and positioning for the instrumentation tube. The grid assembly usually consists of individual strips that interlock to form a lattice. The resulting square cells provide support for the fuel rods in two perpendicular planes. In general, each plane has three support points: two support arches and one spring. The springs and arches are stamped and formed in the grid strip and thus are integral parts of the grid assembly. The springs exert a controlled force, preset so as to optimally maintain the spring force on the fuel rod over the operating life of the fuel assembly.
Fuel assemblies employing spacer grids with flow deflector vanes have usually been fabricated substantially or entirely of inconel or a zirconium alloy, i.e., zircaloy. The inconel grid employing a brazed intersection connection has the advantage of greater strength because of the higher strength of inconel and because the brazing process bonds the intersection of the strips along its entire length. Brazing also has the advantage of providing little or no obstruction to flow. Due to the increased strength, the strip thickness of an inconel grid can be reduced relative to the zircaloy grid to reduce pressure drop and turbulence in the vicinity of the vanes. Annealed zircaloy has commonly been used as a grid material because of its low neutron capture cross-section. A low neutron capture cross-section makes nuclear fission more efficient, thus making the nuclear reactor operate more economically. However, to achieve a strength equivalent to that of an inconel grid, the strip thickness for a zircaloy grid must be increased, thus creating more turbulence and higher pressure drop. Additionally, the joining of the interlocking zircaloy strips conventionally requires welding which melts some grid material to form a weld nugget. The increased strip thickness and weld nuggets for conventional zircaloy grids increase turbulence and grid pressure drop and reduce the effectiveness of the vanes. Therefore, the DNB performance of a zircaloy grid containing flow vanes is degraded relative to an inconel grid vane design.
Co-pending U.S. Patent application No. 679,784, filed Apr. 3, 1991, the disclosure of which is hereby incorporated by reference, discloses providing intermediate welds to improve the strength of the zircaloy grid. Small welding tabs are formed in the grid strips to facilitate the welding process. The slots may be tapered at their ends to facilitate welding at intermediate vertical locations on the strip; thus improving grid strength. Tests have shown an increase in strength of approximately 15% for each intermediate weld over conventional zircaloy grids without intermediate welds. If required, the grid may be welded at three intermediate points, thus increasing its strength by 45% over prior art grids.